You lack of game design knowledge, limitation for better mechanics is a basic.
I'll give you popular exemple to educate you since you seems to lack of brain matter,
Pokemon : You can't catch a full HP pokemon.
Minecraft : You can't run infinitely (food)
Node buster : You have a time limitation
Same for a lot of game, you always have something you can't do for any reason, that's what creates other mechanics to find a solution to do it by another way.
Pokemon : You have to hit the pokemon to catch it (without the limitation, you could just instantly catch)
Minecraft : Creates a way to farm food, kill mobs to eat, otherwise if food wasn't here, you would have less gameplay..
Educate yourself kid.